This invention relates to lustrous coated-mica pigments especially useful for pigmenting of casein plastics and to compositions comprising same.
Casein plastics dating back to 1900 are employed for the manufacture of buttons, combs and the like, and are produced by pressing casein pasted on with water, platicizers, coloring agents and other additives. The thus obtained plastics may be hardened by treatment with formaldehyd. A detailed description of the process is given in: Kunststoff-Handbuch, Vieweg-Becker, Volume 3: Abgewandelte Kunststoffe, Hanser-Verlag, Munich 1965.
The synthetic resin mass can be colored by the incorporation of colored pigments thereon. Whereas especially interesting effects can be generally achieved in the case of coloring plastics with lustrous pigments, the lustrous pigments commercially available heretofore are not satisfactory for incorporating into casein plastics. The lead salts: lead arsenate and lead phosphate, while compatible, are extremely poisonous. Basic lead carbonate is also poisonous and, in addition, is insufficiently chemically stable. Bismuth oxy chloride and natural fish guanine are also not very suitable since both kinds of lustrous pigments are relatively instable under casein-plastic production conditions.
Because of these unsatisfactory properties of the above-mentioned lustrous pigments, a special interest arose very early in lustrous pigments based on coated mica flakes since these pigments are characterized by high stability in working up, great light fastness, vigorous gloss, high chemical stability and high temperature stability. However, attempts to incorporate these lustrous pigments into casein plastics did not give satisfactory results. The principal problem was in deleterious crack and pore formation in the case of working up of buttons and the like. All attempts to prevent this crack formation by special measures have heretofore remained unsuccessful.